Boundary Layer influences on the Subsonic Near-Wake of a Family of Three-Dimensional Bluff Bodies

BOUNDARY LAYER INFLUENCES ON THE SUBSONIC NEAR-WAKE OF A FAMILY OF THREE-DIMENSIONAL BLUFF BODIES Charles William Alcorn Old Dominion University, 1993 Director: Dr. Colin P. Britcher A study is reported on subsonic bluff body near-wake flows. It has been determined that one family of bluff bodies, namely slanted-base ogive cylinders, can experience either a closed recirculating near-wake, or a longitudinal vortex near-wake depending on the base slant-angle and the Reynolds number. This suggests a dependence of near-wake parameters on the state of the boundary layer ahead of separation. This report addresses the influence of the boundary layer on the near-wake of slanted-base bluff bodies. Experiments were conducted in two facilities, the 6-inch Magnetic Suspension and Balance System (MSBS) at NASA Langley Research Center and the Old Dominion University low-speed wind tunnel. Interference-free drag measurements in the 6-inch MSBS validated previous drag results. Measurements in the ODU facility were made to determine base pressures, wake stagnation point locations, and boundary layer velocity profiles. Furthermore, spectral and cross-spectral analyses of the fluctuating streamwise velocity in the near-wake were performed to determine frequencies and coherence of large-scale structures. It was determined that despite variations in the boundary layer state, base pressures and wake stagnation point locations correlate with the Reynolds number based on the boundary layer momentum thickness as the independent variable. Variations in the frequency and coherence of large-scale structures were shown to exist with fixed boundary layer transition. A two-dimensional representation of a slanted-base configuration was studied analytically using classical theories and computationally using an existing finite element package. This study confirmed that the sudden changeover in wake structure is a result of flow reattachment onto the slanted-base. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGMENTS The author gratefully acknowledges the support and guidance of Dr. Colin P. Britcher, Advisor for this research. The author also acknowledges the support of the Institute for Computational and Applied Mechanics (ICAM), Professor S. N. Tiwari, Director. Further acknowledgments are extended to Timothy Schott for his technical support at the NASA Langley 6-inch MSBS, and to Jerry and Bill for help in fabricating the wind tunnel models. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS PAGE LIST OF TABLES..................................................................................... v LIST OF FIGURES................................................................................... vi NOMENCLATURE................................................................................... xi Chapter